A contrast-based technique has been known as one of various techniques for performing the autofocus (AF) function. The contrast-based technique is often adopted in camcorders for general consumers and compact digital still cameras, of which the lens is non-interchangeable. This is because according to the contrast-based technique, no AF errors should occur theoretically due to a calibration error of the camera's mechanism (which will also be referred to herein as a “residual mechanical error”). It should be noted that according to the phase difference method that is often employed in conventional interchangeable lenses for single-lens reflex cameras, AF errors could occur according to the precision or the calibration error (i.e., the residual error) of the camera's mechanism.
If the number of pixels of the imaging device built in the camera body varied according to the specifications of each individual camera unit, then the evaluated AF frequency would also vary from one camera unit to another. According to the contrast-based AF function, in a situation where the given lens unit has such optical performance that would produce some spherical aberration, if the spatial frequency changed, then the peak position would also shift, which is one of various known problems with the contrast-based AF function. That is to say, if a combination of the evaluated AF frequency and the magnitude of spherical aberration changed, then the relation between the AF position and the best in-focus position would also change. As a result, the best AF performance would no longer be guaranteed. For that reason, the contrast-based AF function has rarely been adopted in interchangeable lens camera systems.
Under circumstances such as these, Patent Document No. 1 proposes a technique for applying a contrast-based AF system to an interchangeable lens camera system for camcorders. Specifically, Patent Document No. 1 discloses a technique for transmitting focus information and focus position information from a camera body to a lens unit that are designed to exchange data between them to get the AF control done by the lens unit.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-258088        